Oxygen concentrators often are used in place of oxygen canisters or liquefied oxygen to provide oxygen therapy to a patient. Oxygen concentrators function by eliminating nitrogen from room air. Oxygen concentrators include sieves containing nitrogen-absorbing materials such as zeolite. Room air is drawn into the concentrator, compressed and forced through the sieve. The nitrogen is absorbed out of the air resulting in concentrated oxygen flowing out to the patient. A large volume of air is required for an oxygen concentrator to produce an appropriate amount of concentrated oxygen. Filtering out dust and other foreign particles is essential to keeping a concentrator operating at optimum levels. Generally, the concentrator has an opening in the cabinet for the inflow of air into the device. There is an intake filter across that opening to remove large gross contaminants. This filter is easily removed and cleaned on a daily basis. There is a second filter or inlet pre-filter positioned at the inlet into the air compressor. Heretofore, the inlet pre-filters have had minimal filtering surface that restrict flow into the compressor. It takes approximately seventy (70) liters per minute of air to deliver five (5) liters per minute of highly concentrated oxygen. If an inadequate filtering system is used, the volume of air to the sieve beds will be restricted. This can result in sieve failure and reduced performance.